Digital three dimensional manufacturing, also known as digital additive manufacturing, is a process of making a three dimensional solid object of virtually any shape from a digital model. Three dimensional printing is an additive process in which successive layers of material are formed on a substrate in different shapes. Three dimensional printing is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling.
Some printers that produce three dimensional objects use a flat staging area onto which one or more printheads eject ink drops that build layers of material to form an object. The printhead is typically substantially smaller than the stage so either the printhead or the stage are moved in a process direction and cross-process direction, which are orthogonal to one another in the same plane, multiple times to form each layer of the object. Additionally, one of the stage and the printhead need to be moved in a vertical direction relative to the plane of the process/cross-process direction plane to maintain an appropriate distance between the printhead and the object being built.
Provided that the printhead(s) are at least as wide in the cross-process direction as the object to be produced, drop placement control in the cross-process direction is not required if the spacing of the inkjets in the printheads is sufficient to form the object in that direction. If the resolution of the inkjets is less than the resolution needed for formation of the object in the cross process direction, or if the printhead is narrower than the object being produced by the printing, then multiple passes of the printhead and translation of the printhead in the cross process direction are required to fabricate the object. Resolution of the drop placement in the process direction can be controlled by regulating the velocity of translation for either the stage or the printhead. The vertical separation, or height, is typically controlled by maintaining the mass or volume of the material drops ejected by the printhead in some predetermined range. The mass or volume of the drops are usually calibrated to the range during some type of initial manufacturing set-up or prior to the performance of a print job. Alternatively, a planarization technique can be used to remove excess material from a printed layer to ensure the layer does not exceed the desired height. The addition of a planer adds expense to the printer and increases the time needed to produce an object. The drop mass/volume calibration can drift due to several factors, including temperature, building material consistency within a batch of building material, building material consistency from one batch of building material to another batch of building material, cooling rates, print volume density effects, and other operational parameters. Assuring more accurate drop placement in a printer that produces three dimensional objects is desirable.